The overall objective of this research project is to define the biological importance of the cyclic GMP and its related metabolic components with respect to how they are involved in the modulation of cell function induced by hormones and other biological stimuli. The aim of the proposed research is to develop and apply methodology that will provide a means of exploring the possibility that cyclic GMP is not involved merely as a mass action type of allosteric effector. The alternative to be examined is that cyclic GMP formation and/or hydrolysis serves as a key event underlying certain cellular processes by virtue of the biological energy that can be derived from either the free energy and/or protons liberated as a result of these enzymic transformations. Explicit in this proposed mechanism is a tight coupling of biological stimulus to cyclic GMP generation and its hydrolysis and a greater importance of cyclic GMP turnover than changes in its steady state levels. To examine this concept a procedure will be employed for monitoring cGMP (and cAMP) turnover rates and flux of guanosine (and adenosine) nucleotides. The procedure is based upon the unique feature of cyclic nucleotide phosphodiesterase to catalyze the incorporation of l8O from H218O into the alpha-PO4 of GMP and its recycled nucleoside polyphosphates. The l8O incorporated into this position is resistant to chemical and enzymic exchange or degradation. Changes in cyclic GMP turnover will be studied with respect to stimulus-induced alterations in the function of platelets, smooth muscle and cardiac muscle.